1. Field of the Invention
The present invention relates to a printer apparatus of a construction wherein a carriage supporting member which extends in the reciprocal movement direction of the carriage supports at least a portion of the carriage, and particularly to a printer apparatus which is capable of achieving stable running of the aforementioned carriage.
2. Description of the Related Art
Recently, in accordance with the development of personal computers and the like, there is more demand for printer apparatuses to be of high speed, high resolution, and low cost. Of the printer apparatuses, the ink-jet method, wire dot method, and thermal transfer method are examples of printer apparatuses which are widely used and are of low cost. Particularly, ink-jet type printers are more excellent than the other methods in terms of silence, high speed, a high degree of fine printing, color capabilities, and so forth, and the demand thereof is growing rapidly.
This ink-jet printer apparatus does not press the printing medium directly against the printer head, but arranges the printing surface of the printing medium and the printer head so as to be facing another across a certain distance, and ejects ink droplets from the printer head to certain locations on the printing surface of the printing medium. In other words, with a printer apparatus using such a method, by means of maintaining the distance between the printer head and the printing medium at a proper distance thereafter referred to as "head gap"), ink droplets having a constant dot diameter, concentration, and landing properties can be printed to the printing surface of the printing medium.
In recent years, there is more requirement for printing work which involves printing on printing mediums of various materials and thicknesses. The aforementioned head gap changes according to such difference in thickness of printing medium, and thus it is becoming difficult to maintain quality at a constant.
In order to deal with this, for example, Japanese Unexamined Patent Publication No. 3-239844 discloses an arrangement wherein the printing medium is pressed against a transporting roller located upstream from the platen by means of a medium pressing plate, and causing a carriage mounted with the printer head to perform scanning motion in accordance with this medium pressing plate, so that the head-gap is automatically corrected in accordance with the change in thickness of the printing medium.
FIG. 1 shows an example of such a known printer apparatus. Here, directly underneath a platen 1, which is in the form of a plate, is a rotationally driving transporting roller 2, and the printing medium 5 is pressed against the transporting roller 2 by means of a medium pressing plate 3 which is aided by this transporting roller 2 and a pinch roller 4 which is rotatably attached unto this medium pressing plate 3, thus feeding the printing medium 5 out to the side of the platen 1. Also, rotatably attached to a guide shaft 6 which is parallel to the transporting roller 2 is a carriage 8 mounted with a printer head 7, and is arranged such that the carriage 8 performs scanning motion with the printer head 7 following the guide shaft 6. The carriage 8 is in a state of the end thereof being pressed against the medium pressing plate 3 due to its own weight, and thus slides along the medium pressing plate 3. However, the spacing between the medium pressing plate 3 and the transporting roller 2 changes according to change in the thickness of the medium 5, and thus the head gap between the printing surface of the printing medium and the printer head 7 is constantly maintained the same, regardless of the thickness of the printing medium 5.
In the printer apparatus shown in FIG. 1, in the event that the medium pressing plate 3 is formed of a single material, irregularities occur in the pressure applied by the medium pressing plate 3 to the printing medium 5 if there is the slightest bit of bowing or deformation of this medium pressing plate 3 or eccentricity of the transporting roller 2, thus giving rise to incidents such as inhibiting maintaining of the transporting pitch of the printing medium 5 at a constant or the printing medium becoming crooked as to the correct transporting direction.
Accordingly, thought has been given to making the medium pressing plate 3 easily flexible in the direction of the printing medium 5, or separating the medium pressing plate 3 into several parts along a width direction of the printing medium 5. However, in the event that the medium pressing plate 3 is made to be easily flexible in the direction of the printing medium 5, or separated into several parts along the width direction of the printing medium 5, while the transporting precision of the printing medium 5 does improve, an offset occurs between the portion where the printing medium 5 exists between the transporting roller 2 and the medium pressing plate 3 and where it does not exist between these members, as can be understood from FIG. 2 which illustrates the operational concepts of the medium pressing plate 3 in this case. Thus, when the carriage 8 moves from the right side toward the-left side, for example, in the Figure, the carriage 8 becomes hung on this offset portion 9, changing the sliding resistance of the carriage 8 as to the guide shaft 6, causing temporary irregularities in the running precision of the carriage 8 after passing over the offset portion 9, which has been a problem.